It is well know that information can be transmitted by means of a subcarrier contained on FM broadcast radio waves. Most FM broadcast radio stations use the baseband frequencies from 50 Hz to 53 kHz to transmit stereo program material. In such systems, the baseband frequencies from 53 kHz up to the legal maximum are available for the transmission of other information.
U.S. Pat. No. 4,713,808 (Gaskill) shows how paging information can be transmitted on a modulated subcarrier which has a symmetrical spectral shape. Such a system could use subcarrier centered at 66.5 kHz and extending symmetrically from 57 to 76 kHz. It would be desireable to have a world wide compatible paging network which utilizes the technologies described in the above reference patent and using the 57 to 76 kHz band; however, a conflict exists because there is an existing 57 kHz subcarrier on some European FM stations which uses the baseband frequencies from 54.6 kHz to 59.4 kHz.
The present invention provides a technique and system whereby both the 66.5 kHz subcarrier and the 57 kHz subcarrier can coexist on a single FM broadcast signal with only minimal interference from each other.